1. Field of the Invention
The present invention relates to a double nut for use in fastening two nuts against a bolt in order to fix various kinds of machine devices or iron frames of a building or the like. In particular, it relates to an outside lock nut which is overlapped on a fastening nut so as to prevent the fastening nut from being loosened.
2. Description of the Prior Art
A bolt and a nut for use in fastening component parts to various kinds of machine devices in the prior art or a bolt and a nut used in the case of fixing an iron frame of a building or the like are gradually loosened by vibration or the like of a fastened member.
In view of this fact, there have been various kinds of means widely adopted for preventing looseness of such a nut as above. For example, there is provided a “spring washer,” wherein part of the washer is cut and a resilient force is added. In place of this spring washer, it is also possible to apply a “toothed washer,” wherein projections are embossed on a fastening surface. In addition, it is also possible to apply a “pin member” such as a split pin in which a pin is passed through a nut. The pin member is applied such that common through-pass holes are arranged in a threaded part of a bolt and the nut in their radial direction, and the pin is inserted into these through-pass holes. Further, there is provided a “double-nut.” The double nut is operated such that another looseness preventive nut is overlapped on a fastening nut against a bolt and only the outer nut is further fastened forcedly, using a fastening effect between both the nuts and friction between fastening surfaces.
It can be expected that the aforesaid prior art “spring washer” may provide an effect of preventing looseness just after its fastening. However, since its use of longer period causes a resilient force of the spring washer to be decreased and an effect of preventing looseness to be reduced, this prior art nut had a problem that the nut is not suitable for fastening a heavy iron frame or the like. Similarly, although it could be expected that the “toothed washer” may provide an effect of preventing looseness against a small-sized machine device or a fastening component whose material is soft, as compared with a metallic product, like a wooden product, it could not be expected that an effect of preventing looseness is attained for a large-sized metallic machine device or the like.
In addition, the “pin member” is applied such that there are provided through-pass holes common to the threaded part of the bolt and the nut in their radial direction. Accordingly, when another nut is fastened, if an axial center of each of the through-pass holes does not coincide with each other, the pin may not be passed through the through-pass holes. Further, when the nut was fastened against the bolt in such a way that the axial center of each of the through-pass holes coincided with each other, it showed a problem that they were fastened in loosened state, resulting in that they could not be forcedly fastened.
In addition, when the “double nut” was fastened at one location, it was necessary to fasten nuts twice and an operating step for further fastening the outer nut was added, resulting in quite a troublesome fastening step. In particular, a building or the like requiring a large amount of nut fastening operations had a problem that the fastening steps might become a cause to delay a completion of work. Also, the work for confirming it individually whether or not the looseness preventive outer nut was forcedly fastened was troublesome.
In view of the above, the following patent documents disclose the art for preventing a fastening nut from being loosened by fastening the fastening nut by a lock nut.
Patent Document 1: Japanese Utility Model Unexamined Publication No. Sho 55 (1980)-110820
Patent Document 2: Japanese Patent Unexamined Publication No. Hei 10 (1998)-61645
Namely, the patent document 1 discloses a lock nut “integrally formed with projections projecting outwardly from the vicinity of the threaded hole of the end face on the fastening side of the nut, the projections being made of a resilient member at least softer than the nut.” However, this lock nut is the one in which the projections are simply provided on the end face on the fastening side of the nut. Therefore, when the fastening nut is fastened by the lock nut, the projections enter the space between the end face of the fastening nut and the end face on the fastening side of the lock nut, which produces a clearance there, reducing a fastening force of the fastening nut and the lock nut.
In view of the above problem, the present inventor developed a lock nut according to the patent document 2 wherein “a seat surface of a circumferential edge of a threaded hole of the nut is formed with a plurality of projections, wherein the nut is fastened against a fastening nut already fastened to a bolt of a member to be fastened with its projecting side being faced against the former nut, the projections to be deformed between the seat surfaces of both nuts are applied around the threaded portions of the threaded holes of both nuts and the threaded part of the bolt, to enable a looseness of both nuts to be prevented.” When the nut is fastened against the bolt, these projections and their deformed portions are crushed to increase a frictional force caused by biting of the projections into the threaded part of the bolt, improving an effect of preventing looseness.
However, a problem to be solved is also found in this lock nut. Namely, when the fastening nut is fastened with the lock nut, part of the projections formed on the seat surface may enter the space between the seat surface of the fastening nut and the seat surface of the lock nut, which disturbs the effective use of the fastening force of the fastening nut and lock nut.
The present invention has been invented in view of the aforesaid problem and it is an object of the present invention to provide a lock nut in which a force of fastening between the fastening nut and the lock nut is increased.